The monitoring of length changes of an element in response to exposure to an environmental parameter has been classically used to monitor changes in the environment. A fiber's length has been long used to monitor changes in weather conditions, such as in the classic weather predictor having a witch and children in a house, where the change in the length of the fiber causes the witch and children to rotate into and out of the house through a mechanical linkage.
Similarly, thermal expansion has been commonly used to monitor temperature change. One example of such a device is the use of a bimetallic strip to measure temperature changes by monitoring the change in curvature of the strip caused by the differential expansion of the elements of the strip.
While the above techniques offer a convenient way to measure environmental changes, the sensitivity of these techniques is limited by the sensitivity of the instruments that are currently available to measure the incremental changes resulting from the environmental changes. Direct observation or the use of mechanical linkages are reasonably easy to implement, but are relatively insensitive techniques for measuring and/or amplifying length changes. The changes in length from expansion can be measured more accurately by piezoelectric sensors, optical interferometry, semiconductor strain gauges, and many other techniques which increase the sensitivity of the response to the change in the environmental parameter.
However, even the most sensitive of these techniques for measuring expansion cannot measure very small changes, that is, expansions on the order of one part in a trillion. For example, a temperature increase of only 5 millionths of a degree Centigrade in a 1 millimeter length of aluminum creates a thermal expansion of one part in a trillion.